Pollution of air inside buildings or other confined spaces often leads to discomfort for the occupants of the building, and in some cases it may even be a health hazard. Examples of pollutants that often are present in indoor environments are gaseous odors, microorganisms and smoke from tobacco.
Conventional treatment of polluted indoor air is focused on removal of solid particles by filtration, adsorption or electrostatic techniques. However, such removal only transfers the pollutants from one space to another, and eventually the cleaning surface or the like is saturated, whereby the cleaning efficiency is lowered. Furthermore, low molecular gaseous components are difficult to remove with such techniques.
Alternatively, polluted air can be cleaned by use of photocatalytic air-cleaners. Polluted air is then passed over a photocatalytic surface, which is irradiated with light. Pollutants that are adsorbed on the photocatalytic surface are then decomposed into harmless, odorless, and less toxic compounds. The process of photocatalysis is well known in the art, and no detailed description is given herein. One well-known photocatalyst is titanium dioxide (TiO2), and other known photocatalysts are ZnO, CdS, WO3, SnO2, ZrO2, Sb2O4, CeO2 and Fe2O3. In general, the resulting photocatalytic activity of these materials is higher when the irradiated light is of high energy (short wavelength), such as light in the ultraviolet spectrum. WO 96/37291, U.S. Pat. Nos. 5,045,288 and 4,892,712 all show air cleaners utilizing photocatalysts.
Solar irradiation contains ultraviolet radiation, whereby it may be used as irradiation source in a photocatalytic air cleaning system or pollutant decomposition device. EP 0590 477A1 discloses an example of an air cleaning system in the form of a window of double pane type, wherein the air is passed in-between the two window panes and wherein a photocatalyst is arranged in the intermediate space or on one or both of the intermediate window surfaces. However, such existing systems suffer from a number of disadvantages, which significantly lower the cleaning efficiency of the window, which are overcome by the present invention.